The chloride self-exchange flux across the human red cell membrane is rapidly and reversibly inhibited by 10(-4) M furosemide, a potent chloruretic agent. Furosemide reduces the chloride flux at all chloride concentrations and increases the cellular chloride concentration at which the flux is half-maximum. Kinetic analysis of the flux measurements made at several furosemide and chloride concentrations yields a pattern of mixed inhibition with a dissociation constant for the inhibitor-transport mechanism complex of 5 X 10(-5) M. From this pattern of inhibition and other observations, including that the percent inhibition is independent of pH (range 5.6-8.9), we conclude that the anionic form of furosemide interacts primarily with the chloride transport mechanism at a site separate from both the transport site and the halide-reactive modifier site.

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